6.3.2 Transverse Mercator (-Jt -JT)

The transverse Mercator was invented by Lambert in 1772. In this projection the cylinder touches a meridian along which there is no distortion. The distortion increases away from the central meridian and goes to infinity at 90$^{o}$ from center. The central meridian, each meridian 90$^{o}$ away from the center, and equator are straight lines; other parallels and meridians are complex curves. The projection is defined by specifying:

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The central meridian

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The latitude of origin

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Scale along the equator in inch/degree or 1:xxxxx (-Jt), or map width (-JT)

The optional latitude of origin defaults to Equator if not specified. Although defaulting to 1, you can change the map scale factor via the MAP_SCALE_FACTOR parameter. Our example shows a transverse Mercator map of south-east Europe and the Middle East with 35$^{o}$E as the central meridian:

pscoast -R20/30/50/45r -Jt35/0.18i -B10g5 -Dl -A250 -Glightgray -W0.25p -P > GMT_transverse_merc.ps

Figure 6.14: Rectangular Transverse Mercator map.

The transverse Mercator can also be used to generate a global map--the equivalent of the 360$^{o}$ Mercator map. Using the command

pscoast -R0/360/-80/80 -JT330/-45/3.5i -B30g15/15g15WSne -Dc -A2000 -Gblack -P > GMT_TM.ps

we made the map illustrated in Figure 6.15. Note that when a world map is given (indicated by -R0/360/s/n), the arguments are interpreted to mean oblique degrees, i.e., the 360$^{o}$ range is understood to mean the extent of the plot along the central meridian, while the ``south'' and ``north'' values represent how far from the central longitude we want the plot to extend. These values correspond to latitudes in the regular Mercator projection and must therefore be less than 90 degrees.

Figure 6.15: A global Transverse Mercator map.